The present invention relates to an alignment mark for a light-emitting diode array (referred to below as a LED array), a method of forming the alignment mark, a combined mask employed in this method to form both the alignment mark and the light-emitting diffusion areas in the LED array, and a method of using the alignment mark to align a LED array.
LED arrays can be employed as light sources in electrophotographic printing. Since a linear array of more than a thousand light-emitting elements may be required, the array is normally divided into a plurality of LED array chips, each chip containing part of the array. The LED array chips, and the driver chips containing the LED-array driving circuitry, are bonded to a printed circuit board or other supporting surface. Alternatively, the driver chips are bonded to the printed circuit board, and the LED array chips are piggy-backed onto the driver chips, to save space.
For good printing quality, the light-emitting elements in the array must be evenly spaced. For a printer that prints six hundred dots per inch (600 dpi), for example, the light-emitting elements must be spaced at a pitch of 42.3 micrometers (.mu.m), with a tolerance of substantially .+-.10 .mu.m. This requirement also applies to the light-emitting elements at the ends of adjacent LED array chips. Thus the LED array chips need to be aligned very accurately with one another. For this purpose, alignment marks are necessary on the LED array chips.
A typical LED array chip comprises a gallium-arsenide (GaAs) substrate with a gallium-arsenide-phosphide (GaAsP) epitaxial layer, in which the light-emitting elements are formed by diffusion of a zinc impurity through windows in a diffusion mask. Electrodes of a metal such as aluminum are then formed in a separate step, using a separate mask.
One known method of forming alignment marks is to provide additional windows in the diffusion mask, in which case the alignment marks consist of the same diffused impurity material (zinc) as the light-emitting elements. A disadvantage of this method is that the alignment marks have low contrast and are difficult to detect accurately.
Another known method is to form the alignment marks during the electrode-formation step, in which case the alignment marks comprise the same material (e.g. aluminum) as the electrodes. Alignment marks with good contrast can be obtained in this way, but since the light-emitting diffusions and alignment marks are formed in separate steps, the alignment marks may not be accurately positioned in relation to the light-emitting elements. This is particularly true in the usual case in which a large number of LED array chips are fabricated on a single GaAs wafer. Growth of the GaAsP epitaxial layer tends to warp the wafer, making it practically impossible to align the diffusion mask and the electrode-formation mask accurately over the entire wafer surface.
If the LED array chips are piggy-backed onto the driver chips, first each LED array chip is attached to its driver chip to form a module, then the modules are bonded onto the printed circuit board, so alignment marks are necessary on both the LED array chips and driver chips. To ensure adequate contrast, the alignment marks on the LED array chips conventionally comprise electrode metal as in the second method described above, even though that means accepting a certain degree of misalignment when the modules are assembled. When the modules are bonded to the printed circuit board, they are aligned by means of the alignment marks on the driver chips, and further misalignment tends to occur due, for example, to slippage at the adhesive interface in the bonding process. Thus an accurately aligned LED array is difficult to achieve.